By Tanvi Mehta NEW DELHI (Reuters) -Toxic smog engulfed much of northern India on Monday and pollution readings in New Delhi hit their highest of this pre-winter season, after dense fog overnight. Visibility dropped to 100 metres in the capital and at least eleven flights were diverted.

A thick, toxic smog has enveloped India's capital, home to more than 33 million people. One technology firm which monitors air pollution ranked New Delhi as the most polluted city in the world.

In several areas, pollution levels were more than 50 times higher than the World Health Organization's recommended safe limit.

A thick blanket of toxic smog engulfed most parts of northern India on Monday and readings of air quality in the capital New Delhi hit their highest this year after dense fog overnight.

Pollution in New Delhi shot up to its worst level of the year so far, prompting local authorities to enact emergency measures, including school shutdowns and a ban on construction work.

India said on Sunday that it would tighten restrictions including on construction activities and vehicle movements in New Delhi and surrounding areas from Monday to combat worsening air quality.

India's capital New Delhi switched schools to online classes Monday until further notice as worsening toxic smog surged past 60 times the World Health Organization's recommended daily maximum.- 'Stay indoors' - Primary schools were ordered to cease in-person classes on Thursday,

India's capital New Delhi switched schools to online classes Monday until further notice because of worsening toxic smog, the latest bid to ease the sprawling megacity's health crisis.

The Supreme Court of India has issued an order for the closure of schools for classes 10 and 12 across Delhi-NCR due to the depleting air quality in the area. The court when issuing the order emphasized the need for all educational institutions to switch to online classes as a precautionary measure against hazardous air pollution levels.